This invention relates to a hydrometallurgical process for recovering metal values from sulfidic minerals and more particularly to a leaching process wherein a nitrogen dioxide containing gas is passed through an acidic leaching liquor.
As a preliminary step to winning copper, silver, nickel, cobalt, molybdenum, iron and/or zinc metal from sulfidic minerals, it is necessary to oxidize the minerals to provide aqueous solutions containing ions of these metal values. The oxidation reactions may be effected by high temperature roasts or hydrometallurgical processes involving the use of an aqueous leach liquor. The former procedure has the advantages of being relatively nonselective in terms of mineral reactivities and of consuming significantly less energy during the oxidation process as compared with hydrometallurgical techniques. Unfortunately, conventional roast reactors produce large volumes of dilute sulfur dioxide gas which necessitates the use of expensive ancillary equipment for removing sulfur dioxide from the effluents so that environmental protection standards can be met. The hydrometallurgical processes, on the other hand, typically involve relatively expensive reagents such as nitric acid which, from a practical point of view, must be regenerated and recycled. The cost of the equipment needed to regenerate nitric acid in a conventional nitric acid leach liquor represents a significant capital investment. Accordingly, the use of nitric acid for leaching metal values from sulfidic minerals has not had wide commercial success.